Patients will be provided with spinal anaesthesia for the operation with 2.4 ml 0.5% heavy Marcaine will be administrated into spinal space after position confirmed with spinal needle. No intrathecal opioid will be given during the procedure. Patient will be sedated with TCI propofol (0.3 -1 ug/ml) throughout the whole total knee replacement after successful spinal anaesthesia.